Airguns are used to generate a pressure signal used in marine seismic exploration. Airguns can be described as a chamber with compressed gas that is released to the surrounding water through port(s). The port is the conduit through which the compressed gas escapes the chamber and enters the surrounding water creating an acoustic pulse. The gas release mechanism is similar in all commercial airguns. The gas release mechanism comprises a chamber closed with a lid (shuttle). The shuttle, while moving during firing the airgun (the gas-release cycle), reveals a port exit area as a function of time.
The prior art airguns and methods of operation emit significant acoustic amplitude outside the frequency range of interest for seismic exploration. The out of band signal is considered noise, and may also adversely affect marine life.
It has been determined that the origin of the high frequency energy is the steep rise in the primary pressure pulse. Commonly, these prior art airguns have a linear rate of exposure of the port exit area during the gas-release cycle. In some prior art methods, a secondary release of pressurized gas is utilized to suppress the bubble oscillation in an effort to reduce the excessive noise.
Accordingly, there is a need to provide an airgun and a method that reduces amplitude of the out of band frequencies emitted. There is a further need to provide a system that reduces the steep rise of the primary pressure pulse.